The present invention relates to a process for the removal of alkynes and/or dienes from gas or liquid streams of olefin-containing hydrocarbons, e.g., those derived from steam cracking or refinery processes.
The manufacture of unsaturated hydrocarbons usually involves cracking various types of hydrocarbons and often produces a crude product containing hydrocarbon impurities that are more unsaturated than the desired product. These highly unsaturated hydrocarbon impurities are often very difficult to separate by fractionation from the desired olefin product. The most common example is ethylene manufacture, in which alkynes are common by-products. For example, the effluent from steam or thermal cracking processes for the production of ethylene typically contains, as unwanted impurities, significant amounts of acetylene and C3 to C6 diolefins and acetylenics. Acetylene is difficult to separate from ethylene by fractionation, and conversion by hydrogenation is usually accompanied by a substantial amount of ethylene conversion to ethane. In a similar way, hydrogenation of C3H4(methyl acetylene or allene), propadiene and butadiene results in the production of their olefin analogs, but also significant production of C3 and/or C4 paraffins as a result of over reaction. It has often been difficult industrially to remove such undesirable, highly unsaturated hydrocarbons by hydrogenation so that no significant hydrogenation of desired olefin hydrocarbon takes place.
Two general types of selective hydrogenation processes for removing undesired, unsaturated hydrocarbons have come into use. One, known as “front-end” hydrogenation, involves passing the crude gas in vapor phase from the initial cracking step, after removal of steam and condensable organic material, over a hydrogenation catalyst. This gas typically contains substantial quantities of hydrogen as a result of the cracking step. “Front End” is characterized as hydrogenation before hydrogen has been removed from the balance of the hydrocarbon gas. Despite the large hydrogen content of such gas, which is very greatly in excess of the amount necessary to hydrogenate the alkynes and, therefore, sufficient to hydrogenate a substantial part of the olefin present, operation with sufficient selectivity to produce olefins of polymerization quality is well established and catalyst lives of many years are obtained. In addition, there is a “front end” application involving a catalytic distillation unit and a vapor phase reactor system where the reaction occurs both in the vapor and liquid phases.
In the other type of selective hydrogenation, known as “tail-end” hydrogenation, the crude gas is fractionated and the resulting concentrated product streams are individually reacted with removed hydrogen in a slight excess over the quantity required for hydrogenation of the highly unsaturated hydrocarbons which are present. This process can occur in either the gas or liquid phase dependent upon the pressures utilized. By controlling the amount of hydrogen, the reaction selectivity to olefins can be maximized. However, this requires a multiplicity of reaction systems since following fractionation, there are individual streams of C2's (ethylene, ethane and acetylene), C3's (propylene, propane, methyl acetylene, and propadiene), C4's (butenes, butadiene, Ethyl acetylene, vinyl acetylene, and butanes), each requiring a reactor system. This results in increased capital and operating costs.